In general, magnetic disk drives employ a rotary type of actuator assembly. A voice coil motor (VCM) is constructed of a voice coil mounted on the actuator assembly, a VCM magnet that supplies a magnetic flux space to the voice coil, and a VCM yoke that forms a magnetic path. A processor controls the operation of the actuator assembly by regulating the direction and magnitude of an electric current to be supplied to the voice coil. Servo information that a magnetic head has read from a magnetic disk is utilized to control the actuator assembly. Hereinafter, the control of the actuator assembly that utilizes the servo information is referred to as servo control.
In order to prevent the slider of a magnetic disk drive from sticking to and generating stiction on the surface of a magnetic disk whose rotation has stopped, a magnetic head needs to be unloaded from the recording surface of the magnetic disk while the rotation of the disk is in the stopped state. A load/unload system that utilizes a ramp member disposed near the outer marginal edge of the magnetic disk is employed as one such unloading technique. In the load/unload system, while a lift tab protruding from the leading end of the actuator assembly is in engagement with the sliding surface of the ramp member, the actuator assembly rotationally moves to its home position at which the magnetic head finally stops during unloading.
Also, some magnetic disk drives employs a magnetic latch scheme in which the actuator assembly is magnetically attracted at and fixed to its home position by a strong magnet to prevent the magnetic head from moving to a position above the magnetic disk if an external shock or vibration is applied to the magnetic disk drive with the unloaded magnetic head remaining on the ramp member. The magnetic latch scheme, compared with an inertial latch scheme, is simple in construction and has the advantage that the magnetic disk drive can be manufactured inexpensively. Therefore, the magnetic latch scheme is most commonly used in magnetic disk drives designed for general consumer products, in particular.
While the magnetic head of the magnetic disk drive is flying above the magnetic disk, the operation of the actuator assembly can be servo-controlled by reading written servo data from the magnetic disk and calculating the position and the moving speed of the magnetic head. However, the magnetic head cannot read servo data while the lift tab is moving along the sliding surface of the ramp member (hereinafter, this state is called the load/unload state). In the load/unload state, the voice coil mounted on the actuator assembly controls the rotating speed of the actuator assembly to a constant speed by utilizing the back electromotive force occurring in the voice coil when the coil moves within the magnetic flux space. The back electromotive force, since it is proportional to the moving speed of the voice coil, is hereinafter referred to as the speed electromotive force.
For the magnet latch type of magnetic disk drive, since the actuator assembly undergoes the strong attraction force of the magnet in the load/unload state, the actuator assembly needs to be released from the latch magnet or speed-controlled in defiance of the attraction force. This is why a large current is supplied to the voice coil to give a strong torque thereto. A VCM driver supplies the current of a desired value to the voice coil by conducting current switching control to regulate the torque to be applied to the voice coil. In recent years, more silent operation of the magnetic disk drives in products such as the HDD recorders used in living rooms, is being increasingly demanded, which poses the problem of noise from these magnetic disk drives due to VCM driver switching operation.
Japanese Laid-Open Patent No. 2000-123502 (“Patent Document 1”) discloses a hard-disk drive in which a transducer moves transversely above the surface of a disk in such a way as to form a sine-wave acceleration path. It is described in Patent Document 1 that acoustic noise from a head gimbal assembly is minimized by reducing the higher-harmonic components of rectangular waves using the sine-wave acceleration path. Japanese Laid-Open Patent No. 2001-43645 (“Patent Document 2”) discloses a technique for detecting and controlling the back electromotive force occurring in a VCM. This latter technique uses the detection and control of the back electromotive force within the VCM because, during loading/unloading control of a head, since the head moves along the surface of a ramp while staying away from a disk, servo information present on the disk is neither detectable nor controllable by using normal servo control with the head.